A HEAT EXCHANGER

Abstract: A condensing boiler (1) comprises a heat exchanger (5) having a cylindrical first heat exchange wall (6) surrounded by an outer water chamber (10). A plug (17) extends into the first heat exchange wall (6) and forms a combustion chamber (22) and an annular downwardly extending flue gas passageway (23) for accommodating flue gases from the combustion chamber (22). The plug (17) comprises an outer shell (18) which forms a second heat exchange wall (19) and an inner cylindrical shell (35) which forms with the outer shell (18) an inner water chamber (36). A connecting pipe (46) connects the inner water chamber (36) to the outer water chamber (10) so that water flows in series through the inner water chamber (36) and the outer water chamber (10) from a return water inlet (42) in the inner water chamber (36) to a flow water outlet (16) from the outer water chamber (10).

Description: "A heat exchanger" The present invention relates to a heat exchanger, and in particular, to a heat exchanger for a boiler, such as, for example, a condensing boiler, and in particular, though not limited to a gas fired condensing boiler. The invention also relates to a boiler comprising the heat exchanger. Condensing boilers are known. In general, condensing boilers comprise a heat exchanger, which comprises a pair of spaced apart co-axially arranged cylinders, namely, an inner cylindrical heat exchange wall and an outer cylindrical wall which between them define a cylindrical water chamber. The inner cylindrical heat exchange wall defines a main cylindrical chamber which forms a combustion chamber towards its upstream end and a flue gas passageway extending downwardly from the combustion chamber towards the downstream end for accommodating flue gases from the combustion chamber. Such heat exchangers are generally arranged with the inner and outer cylindrical walls extending substantially vertical with the upstream end at the top and the downstream end at the bottom. A burner which comprises one, or more, and in general, many outlet openings is mounted adjacent the upstream end of the combustion chamber, and is arranged so that the outlet openings deliver jets of a gas/air mixture, which are ignited in the combustion chamber. Flue gases flow downwardly through the flue gas passageway. A motor driven fan blows the gas/air mixture into the burner at a pressure which is sufficient to overcome natural convection and the resistance of the flue gas passageway for urging the flue gases downwardly through the flue gas passageway. A flue gas outlet is normally provided at the downstream end of the flue gas passageway. A return water inlet feeds water into the cylindrical water chamber at the bottom downstream end of the heat exchanger, and a flow water outlet at the upstream end of the heat exchanger delivers heated water from the water chamber. Heat exchange fins may extend inwardly into the main cylindrical chamber from the inner heat exchange wall for conducting heat from the combustion chamber and the flue gases to water in the water chamber. Typical condensing boilers are disclosed in U.S. Patent Specifications Nos. 4,584,968 and 4,589,374 and in British Patent Specification No. 1,263,845. The condensing boilers disclosed in these three patent specifications follow the general construction described above. In the condensing boilers of U.S. Patent Specification No. 4,584,968 a solid plug is located in the lower downstream end of the main cylindrical chamber, and the plug forms with the inner cylindrical heat exchange wall an annular flue gas passageway. A top portion of the plug forms with an upper portion of the inner cylindrical heat exchange wall a combustion chamber. Flue gases from the combustion chamber pass through the annular flue gas passageway formed between the plug and the inner heat exchange wall. Heat exchange fins extend radially inwardly from the inner heat exchange wall into the combustion chamber and into the flue gas passageway and engage the plug. Heat is transferred from the combustion chamber and from the flue gases through the fins and the inner heat exchange wall to water in the water chamber. The provision of the plug regulates the flow of flue gases from the combustion chamber, and prevents the rapid escape of the flue gases, thereby increasing heat transfer from the flue gases to the water chamber. The condensing boiler of U.S. Patent Specification No. 4,589,374 is also provided with a plug located in a downstream portion of the main cylindrical chamber. In this condensing boiler the inner cylindrical heat exchange wall is of helical shape and the plug is a tight fit against the ridges of the inner cylindrical heat exchange wall which forms the helix. Thus, the plug regulates the flow of flue gases from the combustion chamber and causes the flue gases to flow along the helical path formed in the inner cylindrical heat exchange wall. However, other than slowing down the flow of flue gases from the combustion chamber through the flue gas passageway, the provision of the plug in the condensing boilers which are disclosed in U.S. Patent Specifications Nos. 4,589,374 and 4,584,968 does not increase the surface area available for heat transfer to the water. Thus, any efficiency improvements achieved by the provision of a plug in these two boilers is limited. British Patent Specification No. 1,263,845 also discloses a condensing boiler within which a plug is located in the downstream portion of the main cylindrical chamber. The plug of this condensing boiler is of hollow construction and is internally jacketed with an inner water chamber. The inner water chamber is connected to the outer water chamber surrounding the inner cylindrical heat exchange wall by two connections so that the water flowing through the boiler flows in parallel through the inner and outer water chambers. One of the connections between the inner and outer water chambers is at the bottom of the two water chambers adjacent the downstream end of the heat exchanger which accommodates flow of water from the outer water chamber to the inner water chamber. The other of the two connections is adjacent the top of the plug which accommodates the flow of water from the inner water chamber to the outer water chamber. While the condensing boiler of British Specification No. 1,263,845 increases the heat exchange surface area available for transferring heat from the flue gases to the water in the water chambers, nonetheless, by virtue of the fact that the inner and outer water chambers are connected in parallel the increase in heat transfer efficiency which can be achieved from this condensing boiler is limited. Furthermore, heat transfer into the water in the inner water chamber can only be achieved from heat from the flue gases passing through the annular flue gas passageway. Additionally, it has been found that the provision of a plug towards the downstream end of the main cylindrical chamber of such condensing boilers causes two serious problems. Firstly, since the plug, in general, defines the downstream end of the combustion chamber, combusting gases impinge on the upstream end of the plug, thereby causing the plug to become exceedingly hot, and in particular, causing the upstream end of the plug which forms a boundary with the combustion chamber to become excessively hot. This can lead to excessively high temperatures in the combustion chamber which can cause deterioration and failure of the burner. These high temperatures in many cases, can also cause deterioration and disintegration of the plug. A further serious problem which has been found to arise as a result of the provision of such plugs is that the excessive temperature of the plug significantly increases the temperature in the combustion chamber, which in turn causes localised boiling of water in the water chamber jacketing the inner cylindrical heat exchange wall adjacent the combustion chamber. Localised boiling leads to pitting, corrosion and subsequent perforation of the inner cylindrical heat exchange wall. A further problem has been found to arise with the provision of the plugs in the boilers of U.S. Patent Specifications Nos. 4,584,968 and 4,589,374. In the boiler of the former U.S. Specification the heat exchange fins engage the plug, and in the boiler of the latter U.S. Specification the inner heat exchange wall engages the plug directly. Because the plug becomes exceedingly hot, excessive heat is transferred from the plug into the inner cylindrical heat exchange wall either directly or through the heat exchange fins. This excessive heat causes localised boiling in the water chamber, thus, leading to pitting corrosion and perforation of the inner cylindrical heat exchange wall adjacent the flue gas passageway. Indeed, the portions of the fins and/or inner cylindrical heat exchange wall which engage the plug may begin to deteriorate as a result of pitting and corrosion which is directly caused by direct contact with the plug. While the provision of an inner water chamber in the condensing boiler disclosed in British Patent Specification No. 1,263,845 to some extent reduces the possibility of pitting and corrosion in the inner cylindrical heat exchange wall towards the downstream end of the main cylindrical chamber, nonetheless, it does not overcome the problem of pitting and corrosion of the inner cylindrical heat exchange wall adjacent the combustion chamber, and furthermore, it is believed that the excessive heat accumulated in the upstream portion of the plug adjacent the combustion chamber of the boiler disclosed in this British Specification may also lead to pitting and corrosion and subsequent perforation of the walls of the plug which form the inner water chamber due to excessive and localised heat transfer from the top of the plug. It is an object of the invention to provide a heat exchanger for a boiler with improved heat transfer efficiency, and in which significantly more of the available heat in the flue gases is transferred to the water than has been achievable heretofore. It is also an object of the invention to provide a heat exchanger in which pitting and corrosion of the heat exchange walls is minimised. It is also an object of the invention to provide a boiler which comprises such a heat exchanger. The present invention is directed towards providing such a heat exchanger and a boiler. According to the invention there is provided a heat exchanger for a boiler, the heat exchanger having an upstream end and a downstream end and comprising a first heat exchange wall extending around and defining an elongated main chamber having an upstream end and an axial ly spaced apart downstream end corresponding to the upstream and downstream ends, respectively, of the heat exchanger, a hollow elongated plug located in a downstream portion of the main chamber and extending in a generally axial direction therein, the plug comprising a second heat exchange wall forming with the first heat exchange wall a combustion chamber towards the upstream end of the main chamber and an annular flue gas passageway in the downstream portion for accommodating flue gases from the combustion chamber towards the downstream end of the main chamber to a flue gas outlet, a mounting means for mounting a burner in the combustion chamber, an inner water chamber, one wall of which is formed by at least a portion of the second heat exchange wall for transferring heat to water in the inner water chamber, an outer water chamber, one wall of which is formed by at least a portion of the first heat exchange wall for transferring heat to water in the outer water chamber, a return water inlet for delivering water to the heat exchanger to be heated, the return water inlet being provided to the inner water chamber adjacent the downstream end of the heat exchanger, a flow water outlet for delivering heated water from the heat exchanger, the flow water outlet being provided from the outer water chamber towards the upstream end of the heat exchanger, an intermediate water outlet from the inner water chamber, an intermediate water inlet to the outer water chamber, and a connecting means for connecting the intermediate water outlet to the intermediate water inlet for interconnecting the inner and outer water chambers, wherein the intermediate water outlet from the inner water chamber is located near the combustion chamber, and the intermediate water inlet to the outer water chamber is located towards the downstream end of the heat exchanger, and the connecting means connects the intermediate water outlet to the intermediate water inlet so that water flows through the inner and outer water chambers in series from the return water inlet to the flow water outlet. The advantages of the invention are many. A particularly important advantage of the invention is achieved by virtue of the fact that the inner and outer water chambers are connected so that the water flows through the heat exchanger from the return water inlet to the flow water outlet in series, first through the inner water chamber, and then through the outer water chamber. By virtue of the fact that the water flows through the inner and outer water chambers in series significantly improved heat transfer to the water is achieved from the combustion chamber and from the flue gases in the flue gas passageway. This, accordingly, significantly increases the efficiency of the boiler. Preferably, the outer water chamber extends completely around the first heat exchange wall for maximising the heat transfer area between the combustion chamber and the flue gas passageway on the one hand, and the outer water chamber on the other hand. Advantageously, the outer water chamber extends substantially the length of the first heat exchange wall from the upstream end to the downstream end thereof for further maximising the heat transfer area between the combustion chamber and the flue gas passageway on the one hand and the outer water chamber on the other hand. In one aspect of the invention a main outer side wall extends around and is spaced apart from the first heat exchange wall to form with the first heat exchange wall the outer water chamber. Ideally, the width of the outer water chamber between the first heat exchange wall and the main outer side wall does not exceed 20 mm. The narrower the width of the outer water chamber, the more efficient will be the heat transfer from the combustion chamber and the flue gas passageway to the outer water chamber. However, there is a minimum width of outer water chamber below which the heat exchanger would become impracticable. Advantageously, the width of the outer water chamber between the first heat exchange wall and the main outer side wall should not exceed 10 mm, and ideally, the width should not exceed 5 mm. Preferably, the flow water outlet extends from the main outer side wall. Advantageously, the intermediate water inlet is provided in the main outer side wall. In another aspect of the invention the intermediate water inlet to the outer water chamber is located at a position which corresponds to a position in the flue gas passageway at which the temperature of the flue gases is not less than the temperature of the water entering the intermediate water inlet for preventing reverse heat transfer from the water to the flue gases. This feature of the invention provides a particularly important advantage, in that a heat exchanger, and in turn a boiler of optimum efficiency is provided. Due to the fact that the intermediate water inlet to the outer water chamber is located at a position relative to the flue gas passageway where the temperature of the flue gases in the flue gas passageway are not less than the temperature of the water entering the outer water chamber, there is no danger of reverse heat transfer from the water in the outer water chamber to the flue gases in the downstream part of the flue gas passageway. Preferably, the intermediate water inlet is located at a position intermediate the combustion chamber and the downstream end of the heat exchanger. In general, the intermediate water inlet to the outer water chamber is located nearer to the downstream end of the heat exchanger than to the combustion chamber. Preferably, the inner water chamber extends substantially over the entire second heat exchange wall, and in particular, the inner water chamber should extend over the portion of the second heat exchange wall which forms part of the combustion chamber for cooling the plug adjacent the combustion chamber. Ideally, the plug comprises an outer shell which forms the second heat exchange wall, and an inner shell mounted within the outer shell and spaced apart therefrom and forming with the outer shell the inner water chamber. This construction of plug provides a particularly efficient construction of heat exchanger by virtue of the fact that the heat transfer area through the second heat exchange wall from the combustion chamber and the flue gas passageway to the inner water chamber is maximised. It is preferable that the width of the inner water chamber between the inner and outer shells does not exceed 20 mm. While the width of the inner water chamber should be as narrow as possible for maximising heat transfer efficiency to the water in the inner water chamber, nonetheless, below a certain width of inner water chamber, the heat exchanger would become impracticable. Preferably, the width of the inner water chamber between the inner and outer shells does not exceed 10 mm, and ideally, the width of the inner water chamber between the inner and outer shells does not exceed 5 mm. In one aspect of the invention the outer shell of the plug comprises an elongated outer side wall extending around the plug and forming with the first heat exchange wall the flue gas passageway, the outer side wall forming an elongated inner cavity, and an outer end wall at the upstream end of the outer side wall closing the inner cavity at the upstream end, the outer end wall forming with the first heat exchange wall the combustion chamber. In another aspect of the invention the inner shell comprises an inner side wall extending around and within the inner cavity, and being co-axial with and spaced apart from the outer side wall, and an inner end wall at the upstream end of the inner side wall which is spaced apart from the outer end wall, the inner side wall and inner end wall defining with the outer side wall and outer end wall the inner water chamber. Preferably, the inner and outer end walls of the plug are of dome shape, and the intermediate water outlet is located adjacent the centre of the dome for further increasing the available heat transfer area between the combustion chamber and the inner water chamber. In another aspect of the invention the intermediate water outlet is provided from the inner shell of the plug, and the return water inlet is provided in the outer shell of the plug. In a further aspect of the invention a plurality of heat exchange fins extend from at least a portion of at least one of the first and second heat exchange walls into the flue gas passageway for transferring heat from the flue gases in the flue gas passageway to the at least one of the outer and inner water chambers. The provision of the heat exchange fins significantly increases the available heat transfer surface area for transferring heat from the flue gases in the flue gas passageway into the water in the respective inner and outer water chambers. Preferably, the heat exchange fins extend longitudinally in a generally upstream/downstream direction, and are spaced apart circumferentially around the at least one of the first and second heat exchange walls. Advantageously, the heat exchange fins are provided on the first and second heat exchange walls. Ideally, the heat exchange fins extend substantially radially from the corresponding heat exchange walls. In one aspect of the invention the heat exchange fins extend from the first and second heat exchange walls and terminate in respective distal edges at positions intermediate the first and second heat exchange walls. Preferably, a clearance is provided between the distal edges of the respective heat exchange fins extending from the opposite heat exchange walls to facilitate removal of the plug from the main chamber. In another aspect of the invention the heat exchange fins are arranged so that the sum of the heat exchange surface areas of the heat exchange fins per unit area of the corresponding first and second heat exchange walls increases from the upstream end of the flue gas passageway to the downstream end thereof. In a further aspect of the invention the sum of the heat exchange surface areas of the heat exchange fins per unit area of the corresponding first and second heat exchange walls increases from the upstream end of the flue gas passageway to the downstream end thereof in a way that the quantity of heat being transferred to the respective first and second heat exchange walls is substantially constant along the length of the respective first and second heat exchange walls from the upstream end thereof to the downstream end thereof. Preferably, the number of the heat exchange fins per unit area of the corresponding first and second heat exchange walls increases from the upstream end of the flue gas passageway to the downstream end thereof. In one aspect of the invention the number of heat exchange fins per unit area of the corresponding first and second heat exchange walls increases at intervals in a downstream direction along the flue gas passageway. Preferably, the heat exchange fins are arranged in sets, each set comprising a plurality of heat exchange fins spaced apart circumferentially around the flue gas passageway, the sets of heat exchange fins being arranged end to end in a generally upstream/downstream direction. Advantageously, the heat exchange fins of at least some of the sets of heat exchange fins are staggered relative to the heat exchange fins of an adjacent set of heat exchange fins to induce turbulence in the flue gases passing through the flue gas passageway. Ideally, each set of heat exchange fins is formed by an elongated endless band of heat conductive material, each band being formed into a waveform, portions of the waveform joining opposite peaks of the waveform forming the heat exchange fins. Preferably, each band is of square wave formation, the heat exchange fins being formed by radially extending portions of the square wave. Preferably, the bands are secured to the respective heat exchange surfaces by the peaks joining roots of the waveform. Ideally, the first heat exchange wall and the plug are of cylindrical construction. Preferably, the plug is located co- axially within the first heat exchange wall. Additionally, the invention provides a boiler comprising the heat exchanger according to the invention. In one aspect of the invention the boiler is a condensing boiler. Preferably, the boiler is gas fired, and ideally, the boiler is suitable for mounting with the upstream end of the heat exchanger towards the top, and the downstream end of the heat exchanger towards the bottom. Advantageously, the burner is mounted in the combustion chamber adjacent the upstream end thereof. Apart from the advantages of the invention discussed above, other important advantages of the invention are achieved in the aspects of the invention where the inner water chamber extends over the portion of the second heat exchange wall of the plug which forms the combustion chamber. In these aspects of the invention, by virtue of the fact that the inner water chamber extends over the portion of the second heat exchange surface which forms part of the combustion chamber, the temperature of the plug is prevented from exceeding an excessively high temperature. This, thus, facilitates in preventing excessively high temperatures in the combustion chamber, which in turn minimises and in most cases eliminates the risk of localised boiling of the water in the outer water chamber adjacent the combustion chamber. Thus, any danger of pitting corrosion and subsequent perforation of the first heat exchange wall is avoided. Similarly, the fact that the inner water chamber prevents excessive heating of the plug, there is little danger of localised boiling of the water in the inner water chamber. Thus, the risk of pitting, corrosion and subsequent perforation of the second heat exchange wall is minimised, and in most cases eliminated. The invention will be more clearly understood from the following description of an embodiment thereof given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a partly cut-away perspective view of a boiler according to the invention, Fig. 2 is a cross-sectional front elevational view of the boiler of Fig. 1, Fig. 3 is a cross-sectional plan view of the boiler on the line III - III of Fig. 2, Fig. 4 is a cross-sectional elevational view of a portion of the boiler of Fig. 1, Fig. 5 is a front elevational view of another portion of the boiler of Fig. 1, Fig. 6 is a perspective view of a detail of the portion of the boiler of Fig. 5, Fig. 7 is a perspective view of another detail of the boiler of Fig. 1, Fig. 8 is a perspective view of a further detail of the boiler of Fig. 1, Fig. 9 is a sectional plan view of a still further detail of the boiler of Fig. 1, Fig. 10 is a view similar to Fig. 9 of another detail of the boiler of Fig. 1, Fig. 11 is a view similar to Fig. 9 of a still further detail of the boiler of Fig. 1, Fig. 12 is a perspective view of portion of the detail of Fig. 11, Fig. 13 is a typical cross-sectional plan view of a portion of the boiler of Fig. 1, Fig. 14 is a sectional front elevational view of a portion of the boiler of Fig. 1, Fig. 15 is a sectional front elevational view of another portion of the boiler of Fig. 1, Fig. 16 is a sectional plan view of the boiler on the lines XVI-XVI of Fig. 2, and Fig. 17 is a cross-sectional front elevational view of a boiler which is substantially similar to the boiler of Fig. 1. Referring to the drawings and initially to Figs. 1 to 16 there is illustrated a boiler according to the invention indicated generally by the reference numeral 1. In this case the boiler 1 is a condensing boiler. The boiler 1 comprises a heat exchanger also according to the invention and indicated by the reference numeral 5, and a premix gas fired burner unit 76 mounted in the heat exchanger 5 as will be described below. The heat exchanger 5 comprises a main elongated housing 2 of cylindrical shape having an upstream end 3 and a downstream end 4. The main housing 2 comprises an elongated cylindrical first heat exchange wall 6 which forms a main chamber 7 which is described in detail below. A cylindrical main outer side wall 9 extends around the first heat exchange wall 6 and forms with the first heat exchange wall 6 an outer cylindrical water chamber 10. The outer water chamber 10 extends completely around the first heat exchange wall 6 and extends from the upstream end 3 of the heat exchanger 5 to a position 11 spaced apart in an upstream direction from the downstream end 4 of the heat exchanger 5. The first heat exchange wall 6 is shaped at its upstream end and downstream end and is seam welded at 14 and 15, respectively, to the main outer side wall 9 to close the upstream and downstream ends, respectively, of the outer water chamber 10. A flow water outlet 16 extends from the outer water chamber 10 through the main outer side wall 9 adjacent the upstream end thereof for delivering heated water from the boiler 1. An end wall 20 secured to the main outer side wall 9 by a circular vee-clamp 26 closes the main chamber 7 at the upstream end 3. The vee-clamp 26 extends around the periphery of the end wall 20 and engages a circumferential flange 12 extending from the main outer side wall 9 for securing the end wall 20 to the main outer side wall 9. A gasket 13 between the end wall 20 and the flange 12 seals the joint between the end wall 20 and the flange 12. The burner unit 76 which is described below is mounted in the main chamber 7 and carried by the end wall 20. An elongated hollow water cooled cylindrical plug 17 is mounted co-axially in a downstream portion 21 of the main chamber 7 and forms with the first heat exchange wall 6 and the end wall 20 an upstream combustion chamber 22. The plug 17 also forms with the first heat exchange wall 6 an annular flue gas passageway 23 which extends from the combustion chamber 22 in a generally downstream direction for accommodating flue gases from the combustion chamber 22. The plug 17 comprises an outer shell 18 which forms a second heat exchange wall 19. The outer shell 18 is formed by a cylindrical outer wall 37 which terminates at its upstream end in a dome shaped outer end wall 38, both of which form the second heat exchange wall 19. The cylindrical outer wall 37 forms with the first heat exchange wall 6 the flue gas passageway 23, while the outer end wall 38 forms with the first heat exchange wall 6 and the end wall 20, the combustion chamber 22. The outer wall 37 extends from the outer end wall 38 to the downstream end 4 of the heat exchanger 5 and terminates at a position which is spaced apart in a downstream direction from the downstream end 11 of the first heat exchange wall 6 and the outer water chamber 10. A flue gas chamber 27 for receiving flue gases from the flue gas passageway 23 extends around the outer wall 37 at the downstream end thereof. The flue gas chamber 27 is formed by a cylindrical flue gas chamber wall 28 which is spaced apart from and which extends around the outer wall 37. An annular base wall 29, which extends inwardly from the combustion chamber wall 28 to the outer wall 37 closes the flue gas chamber 27. A flange 30 extending from the base wall 29 is continuously seam welded to the outer wall 37 for sealing the flue gas chamber 27. A circumferential flange 32 extending outwardly around the combustion chamber wall 28 is releasably secured by a circular vee-clamp 33 to a corresponding circumferential flange 24 extending around the main outer side wall 9. A gasket 25 between the flanges 32 and 24 seals the joint. Accordingly, the vee-clamp 33 releasably secures and retains the plug 17 axially in the main chamber 7. A flue gas outlet 31 through the flue gas chamber wall 28 delivers flue gases from the flue gas chamber 27. The outer shell 18 of the plug 17 defines an inner cavity 34 within which an inner shell 35 is co-axially located and spaced apart from the outer shell 18 for forming with the outer shell 18 an inner water chamber 36. The inner shell 35 is formed by a cylindrical inner wall 39 which terminates at an upstream end in a dome shaped inner end wall 40. The downstream end of the inner wall 39 is shaped at 41 and continuously seam welded to the outer wall 37 for sealing the inner water chamber 36. Heat from the combustion chamber 22 is transferred into the water in the inner water chamber 36 through the dome shaped outer end wall 38 of the second heat exchange wall 19, and heat from the flue gases passing through the flue gas passageway 23 is transferred into the water in the inner water chamber 36 through the outer wall 37 of the second heat exchange wall 19, as will be described below. Heat is transferred from the combustion chamber 22 and from the flue gases in the flue gas passageway 23 through the first heat exchange wall 6 into the water in the outer water chamber 10 as will also be described below. A return water inlet 42 to the inner water chamber 36 extends from the outer wall 37 of the plug 17 at the downstream end thereof for delivering water to be heated to the boiler 1. An opening 44 sealably accommodates the return water inlet 42 through the flue gas chamber wall 28. An intermediate water outlet 43 from the inner water chamber 36 is provided from the inner end wall 40 adjacent the centre thereof, so that the intermediate water outlet 43 is provided from the inner water chamber 36 near the combustion chamber 22. An intermediate water inlet 45 to the outer water chamber 10 is provided through the main outer side wall 9 towards the downstream end thereof. Although to facilitate illustration, in Fig. 16 the intermediate water inlet 45 is illustrated as being angularly offset relative to the flue gas outlet 31, and in the other Figs, the intermediate water inlet 45 is illustrated as being in the same vertical plane as the flue gas outlet 31, the intermediate water inlet 45 lies in the same vertical plane as the flue gas outlet 31. However, it will be appreciated that the intermediate water inlet 45 and the flue gas outlet 31 may be angularly offset. A connecting means comprising a connecting pipe 46 extending through the inner cavity 34 connects the intermediate water outlet 43 to the intermediate water inlet 45 so that water being heated in the heat exchanger 5 flows in series through the inner water chamber 36 and the outer water chamber 10 from the return water inlet 42 to the flow water outlet 16. A union connection 49 releasably connects the pipe 46 to the intermediate water inlet 45 to facilitate removal of the plug 17 from the main chamber 7. The intermediate water inlet 45 is located adjacent the downstream end of the outer water chamber 10. In other words, the intermediate water inlet 45 is provided at a location which is spaced apart in an upstream direction from the downstream end 4 of the inner water chamber 36, and also at a location which is intermediate the combustion chamber 22 and the downstream end of the flue gas passageway 23 but towards the downstream end of the flue gas passageway 23. Since heat is transferred from the flue gases to the inner water chamber 36 along the second heat exchange wall 19 to the downstream end 4 thereof, the flue gas chamber is considered to be part of the flue gas passageway 23. The location of the intermediate water inlet 45 is arranged so that it corresponds to a position in the flue gas passageway 23 at which the temperature of the flue gases is not less than the temperature of the water entering through the intermediate water inlet 45. This prevents loss of heat to the flue gases from the water which has already been heated in the inner water chamber 36 as it enters the outer water chamber 10 through the intermediate water inlet 45. In this embodiment of the invention the distance <-- of the centre of the intermediate water inlet 45 from the downstream end 4 of the inner water chamber 36 is approximately 120 mm, see Fig. 2. For convenience and to avoid a dead spot in the outer water chamber 10, the outer water chamber 10 terminates at the downstream end adjacent the intermediate water inlet 45. If desired the outer water chamber 10 may extend to the downstream end 4 of the heat exchanger 5. However, it is believed that little heat transfer to the water in the portion of the outer water chamber 10 between the intermediate water outlet 45 and the downstream end 4 of the first heat exchange wall 6 would be achieved. A helical partition 80 is provided in and extends the length of the outer water chamber 10 to form a helical passageway 81 which extends from the intermediate water inlet 45 to the flow water outlet 16 through which the water passes between the intermediate water inlet 45 and the flow water outlet 16. This maximises heat transfer through the main heat exchange wall 6 to the water in the main water chamber 10. The helical partition 80 is formed by an elongated strip 82 of sheet stainless steel, see Fig. 14. The strip 82 is bent at 83 and 84 longitudinally along its length to form longitudinally extending flanges 85 and 86. The flange 85 is welded to the first heat exchange wall 6, and the strip 82 is arranged so that the flange 86 tightly abuts the main outer side wall 9 thereby forming the helical passageway 81. A similar helical partition 88 is provided between the outer shell 18 and inner shell 35 of the plug 17 to form a helical passageway 89 extending through the inner water chamber 36 between the return water inlet 42 and the intermediate water outlet 43. The function and action of the helical passageway 89 in the inner water chamber 36 is similar to that of the passageway 81 in the outer water chamber 10. Additionally, the construction of the helical partition 88 is similar to the helical partition 80, and similar components are identified by the same reference numerals, see Fig. 15. The flange 85 of the helical partition 88 is welded to the inner shell 35 and the flange 86 tightly abuts the outer shell 18. The first heat exchange wall 6 defines a first heat exchange surface 50 which receives heat from the combustion chamber 22 and the flue gas passageway 23 for transfer into the water in the outer water chamber 10. The second heat exchange wall 19 formed by the outer cylindrical wall 37 and the outer end wall 38 of the plug 17 defines a second heat exchange surface 51 which receives heat from the flue gas passageway 23 and the combustion chamber 22 for transfer to the inner water chamber 36. A plurality of sets 52a to d of heat exchange fins 53 extend from the second heat exchange surface 51 formed by the outer cylindrical wall 37 into the flue gas passageway 23 for transferring heat from the flue gases into the outer wall 37. Each set 52 of fins 53 are formed from an endless band 54 of heat conductive material, in this case, stainless steel, see Figs. 3 to 12. The bands 54 which form the sets 52 of fins 53 are shaped to form a square waveform. Portions 55 of the bands 54 extending between opposite peaks 59 of the waveform form the fins 53. Portions 56 of the bands 54 which form one of the peaks 59 join the roots of the fins 53. Portions 57 of the bands 54 which form the other peaks 59 join the distal edges 48 of the fins 53. Each band 54 of fins 53 extends around the outer wall 37 and is secured to the outer wall 37 by braising the portions 56 to the second heat exchange surface 51. The sets 52 of fins 53 are arranged so that the fins 53 of each set 52 extend in a generally axial direction along the flue gas passageway 23, and extend radially outwardly from the second heat exchange surface 51 towards the first heat exchange surface 50. In this embodiment of the invention the heat exchange fins 53 terminate along their distal edges 48 at positions intermediate the first heat exchange surface 50 and the second heat exchange surface 51 as will be described in more detail below. The bands 54 forming the sets 52-6 to d are formed so that the portions 55 forming adjacent fins 53 are spaced apart circumferentially so that each fin 53 defines two heat exchange surfaces 47 for receiving heat from the flue gases. The bands 52a are formed so that portions 55 forming the fins 53 which are joined at their distal edges 48 abut each other so that only one heat exchange surface 47 of each fin 53 is exposed to the flue gases. In this way, the sum of areas the heat exchange surfaces 47 of the fins 53 of the bands 52a which are provided towards the upstream end of the flue gas passageway 23 per unit area of the second heat exchange surface 51, is considerably less than the sum of the areas of the heat exchange surfaces 47 of the fins 53 of the bands 52/- to d towards the downstream end of the flue gas passageway 23 per unit area of the second heat exchange surface 51. Additionally, the pitch e of the portions 55 of the bands 52 forming the fins 53 reduces in a downstream direction. In other words, the pitch e between the fins 53 of the bands 52d is less than that between the fins 53 of the band 52c and similarly, the pitch e between the fins 53 of the band 52c is less than the pitch of the band 52b thus, the number of fins 53 is greater in the band 52d than in the band 52c, while the number of fins 53 in the band 52c is greater than that in the number of fins 53 in the band 52b. Furthermore, the number of fins 53 in the band 52b is greater than the number of fins 53 in the band 52a. This, thus further increases the total sum of areas of the heat exchange surfaces 47 of fins 53 per unit area of the second heat exchange surface 51 in a downstream direction along the flue gas passageway 23. Further, the sum of the areas of the heat exchange surfaces 47 of the fins 53 per unit area of the second heat exchange surface 51 is lowest in the flue gas passageway 23 where the temperature of the flue gases is highest, and the sum or the areas of the heat exchange surfaces 47 per unit area of the second heat exchange surface 51 is greatest in the downstream end of the flue gas passageway 23 where the temperature of the flue gases is lowest. The sum of the areas of the heat exchange surfaces 47 of the fins 53 per unit area of the second heat exchange surface 51 is selected so that the quantity of heat being transferred through the second heat exchange wall 19 over substantially the entire length of the flue gas passageway 23 is maintained reasonably constant. A plurality of sets 60a to c of heat exchange fins 51 extend from the first heat exchange surface 50 of the first heat exchange wall 6 over the length of the first heat exchange wall 6 from a position adjacent the sets 52a to a position adjacent the downstream end 11 of the outer water chamber 10. Each set 60 of fins 61 is formed from an endless band 63 of stainless steel material in similar fashion as the sets 52 of fins 53 are formed from the bands 54. For convenience the portions of the bands 63 which are similar to the bands 54 are identified by the same reference numerals. The bands 63 are secured to the first heat exchange wall 6 by braising the portions 56 joining the roots of the fins 61 to the first heat exchange surface 50. Thus, the heat exchange fins 61 extend longitudinally along the first heat exchange wall 6 and radially outwardly therefrom towards the second heat exchange surface 51. Distal edges 66 of the fins 61 extend to positions intermediate the first and second heat exchange surfaces 50 and 51 and terminate at positions spaced apart from the distal edges 48 of the fin 53. Thus clearance is left between the distal edges 48 and 66 of the fins 53 and 61, respectively, for facilitating removal of the plug 17 from the main chamber 7. In this embodiment of the invention the construction of and the number of fins of the sets 60a of fins 61 are similar to those of the sets 52a of fins 53. The dimensions of the fins 61 of the sets 60a are substantially similar to those of the fins 53 of the sets 52a, allowance being made to accommodate the larger circumference around which the sets 61a must extend. Similarly, the construction number and dimensions of the sets 60b and c of fins 61 are substantially similar to those of the sets 52-6 and c of fins 53, respectively. In this embodiment of the invention two sets of fins 52a and 60a are provided. Six sets of fins 52-6 and 606 are provided, and twelve sets 52c and 60c are provided. Five sets 52d of fins 53 are provided. There are no sets of fins extending from the first heat exchange wall 6 corresponding to the sets 52d of fins 53. As in the case of the fins 53, the sum of the areas of the heat exchange surfaces 69 of the fins 61 per unit area of the first heat exchange surface 50 increases along the flue gas passageway 23 in a downstream direction for increasing the flue gas/heat exchange surface area of the fins 61 as the temperature of the flue gases decreases. The sum of the areas of the heat exchange surfaces 69 of the fins 61 per unit area of the first heat exchange surface 50 is selected so that the quantity of heat being transferred through the first heat exchange wall 6 over substantially the entire length of the flue gas passageway 23 is maintained reasonably constant. To increase turbulence of the flue gases flowing through the flue gas passageway 23, for enhancing heat transfer from the flue gases to the fins 53 and 61, respectively, adjacent sets 52 and 60 of heat exchange fins 53 and 61, respectively, are staggered relative to their adjacent sets 52 and 60, respectively. This can be clearly seen in Figs. 6 and 13. While it is advantageous that all adjacent sets 52 and 60 should be so staggered, in practical assembly this may not be possible. However, it is important that a sufficient number of the sets 52 and 60 should be staggered relative to their adjacent respective sets 52 and 60 to induce sufficient turbulence in the flue gases to optimise heat transfer from the gases into the heat exchange fins 53 and 61, and in turn, into the respective heat exchange walls 19 and 6. In this embodiment of the invention the axial length 7 L of the heat exchanger 5 is 400 mm. The axial length 7 2 of the outer wall 37 of the plug 17 is 250 mm. The radial width w of the flue gas passageway 23 between the first and second heat exchange surfaces 50 and 51 is 10 mm. The diameter of the surface 50 of the first heat exchange wall 6 is 140 mm. The radial width of the outer water chamber 10 between the first heat exchange wall 6 and the main outer side wall 9 is approximately 5 mm, and the radial width of the inner water chamber 36 between the outer shell 18 and the inner shell 35 is approximately 5 mm all round. The walls forming the main housing 2 and the plug 17 are all of stainless steel sheet material of approximately 1.2 mm thickness. The heat exchange fins are of stainless steel sheet material, and the fins 53 and 61 of the sets 52a, 526, 60a and 606 are of thickness 0.4 mm. The fins 53 and 61 of the sets 52c, 52d and 60c are of 0.2 mm thickness. In this embodiment of the invention the bands 54 and 63 of the sets 52a and 60a, respectively, comprise 100 fins. The pitch e of adjacent fins 53 of 8.2 mm. The bands 54 and 63 of the sets 526 and 606 of fins 53 and 61, respectively, comprise 200 fins at a pitch e of 2.1 mm. The bands 54 and 63 of the sets 52c and 60c of fins 53 and 61, respectively, comprise 300 fins at a pitch e of 1.62 mm. The bands 54 of the sets 52c/ comprise 300 fins at a pitch e of 1.4 mm. Each band 54 and 63 is of axial height h of 10 mm. The radial length r of the fins 53 and 61 of the sets 52a and 6 and 60a and 6, respectively, is 3.5 mm. The radial length r of the fins 53 and 61 of the sets 52c and 60c, respectively, is 4.5 mm. The radial length r of the fins 53 of the sets 52d is 9 mm. The total surface area of the fins 53 of the sets 52a is approximately 6,624 mm 2 . The total surface area of the fins 53 of the sets 526 is approximately 89,237 mm 2 . The total surface area of the fins 53 of the sets 52c is approximately 325,195 mm 2 . The total surface area of the fins 53 of the sets 52d is approximately 270,659 mm 2 . The total surface area of the fins 61 of the sets 60a is approximately 7,321 mm 2 . The total surface area of the fins 61 of the sets 606 is approximately 98,630 mm 2 . The total surface area of the fins 61 of the sets 60c is approximately 353,927 mm 2 . The burner unit 76 is mounted in the combustion chamber 22, and comprises a gas chamber 77 into which a mixture of gas and air under pressure is delivered through a gas/air inlet 78. A diaphragm 79 comprising a plurality of gas outlets, namely, perforations (not shown) extends across the gas chamber 77 for accommodating the gas/air mixture into the combustion chamber 22. The combustion of a gas/air mixture in a condensing boiler will be well known to those skilled in the art. A fan (not shown) delivers a pressurised mixture of air and gas through the gas/air inlet 78. The fan is arranged so that air and gas are drawn into the fan. The fan in turn delivers the gas/air mixture under sufficient pressure into the gas chamber 77 of the burner unit 76 so that the flue gases are sufficiently pressurized to overcome natural convection, and flow downwardly through the flue gas passageway 23. Suitable ignition means (not shown) for igniting the gas/air mixture is provided in the combustion chamber 22. Such ignition means will be well known to those skilled in the art. Control circuitry (not shown) is provided for controlling operation of the boiler 1, such control circuitry will be well known to those skilled in the art. Briefly, the control circuitry operates a valve (not shown) for controlling the supply of gas to the fan (not shown). The control circuitry controls a motor (not shown) for driving the fan for delivering the gas/air mixture to the burner unit 76. The control circuitry operates the gas ignition means (not shown) in the combustion chamber 22. A temperature sensor (also not shown) on the main outer side wall 9 adjacent the flow water outlet 16 is connected to the control circuitry which controls the boiler 1 in response to the monitored temperature recorded by the temperature sensor. In use, the boiler 1 is mounted on a suitable mounting, for example, a wall or the like with the housing 2 vertically orientated with the upstream end 3 at the top and the downstream end 4 at the bottom. Return water from a central heating system or other system is connected to the return water inlet 42 and the flow water outlet 16 is connected to the flow water pipe of the central heating system. The flue gas outlet 31 is connected to an exhaust pipe which, in general, would be ducted out of a building in which the boiler is located. An air supply is connected to the inlet of the fan (not shown) as is a gas supply. An electrical power supply is connected to the control circuitry (not shown). The gas/air mixture is ignited in the combustion chamber 22 and flue gases are driven downwardly under pressure by the action of the fan (not shown) through the flue gas passageway 23, and pass into the flue gas chamber 27, and in turn are exhausted through the flue gas outlet 31. Heat is conducted from the combustion chamber 22 through the outer dome shaped end wall 38 of the plug 17 into the inner water chamber 36, and heat is conducted from the combustion chamber 22 through the portion of the first heat exchange wall 6 which forms the combustion chamber 22 into the outer water chamber 10. Heat from the flue gases is transferred through the fins 53 and 61, the outer wall 37 of the plug 17 and the first heat exchange wall 6 into the inner water chamber 36 and the outer water chamber 10, respectively. Water under the action of a circulating pump in the central heating or other system is delivered in series through the inner water chamber 36 first, and in turn, through the outer water chamber 10 and out through the flow water outlet 16. With regard to the fins, it has been found that by leaving the dome shaped outer end wall 38 free of fins and the portion of the first heat exchange wall 6 which forms the combustion chamber 22 also free of fins, and by providing the fins 53 and 61 in the arrangement described on the respective outer wall 37 of plug 17 and the first heat exchange wall 6, the quantity of heat per unit area of the second heat exchange wall 19 being transferred into the inner water chamber 36 is reasonably constant over the entire area of the second heat exchange wall 19, and the quantity of heat per unit area of the first heat exchange wall 6 being transferred into the outer water chamber 10 through the first heat exchange wall 6 is also reasonably constant over the entire area of the first heat exchange wall 6. However, it is not necessary that the quantity of heat per unit area being delivered into the outer water chamber 10 through the first heat exchange wall 6 should be similar to the quantity of heat per unit area being delivered into the inner water chamber 36 through the second heat exchange wall 19, although, in general, the quantities of heat transfer per unit area of the respective first and second heat exchange walls will be reasonably similar. By leaving the portion of the first heat exchange wall which forms part of the combustion chamber free of fins, it has been found that this further minimises the risk of localised boiling of water in the outer water chamber. It is believed that the provision of fins on the portion of the first heat exchange wall which forms the combustion chamber would increase the quantity of heat being transferred from the combustion chamber into the water to such an extent that localised boiling could occur. Additionally, it has been found that the omission of fins on the domed shaped portion of the second heat exchange wall which forms the plug also further facilitates in minimising the risk of localised boiling of water in the inner water chamber. The boiler and heat exchanger according to the invention are particularly efficient. Additionally, by virtue of the arrangement of the fins, and the fact that the available heat exchange surface in contact with the flue gases increases along the flue gas passageway compensates for the fact that the temperature of the flue gas is decreasing in a downstream direction. This thus assists in maintaining the quantity of heat transfer into the water in the respective outer and inner water chambers 10 and 36 substantially constant from the upstream to the downstream ends along the first and second heat exchange walls 6 and 19. Additionally, by virtue of the fact that the inner water chamber extends over substantially the entire area of the second heat exchange surface 19 of the plug 17 a relatively large surface area is provided for transferring heat into the water, thereby significantly increasing the quantity of heat which is extracted from combustion and the flue gases, thereby significantly increasing the efficiency of the heat exchanger. Furthermore, by virtue of the fact that the inner water chamber extends over the dome shaped portion of the plug 17, namely, the portion of the plug 17 which partly forms the combustion chamber 22, this further facilitates in heat transfer from the combusting gases into the water in the inner water chamber. An advantage of releasably mounting the plug 17 in the main chamber 7 by the jubilee clip 33, is that the plug can be readily easily removed from the main chamber 7 to facilitate cleaning of the boiler 1. Additionally, by virtue of the fact that the end wall 20 is releasably connected to the outer wall 9 by the jubilee clip 26, the burner 76 can readily easily be removed from the combustion chamber 22 by removing the end wall 20. This likewise facilitates cleaning and maintenance of the boiler 1. Referring now to Fig. 17 there is illustrated a boiler according to another embodiment of the invention indicated generally by the reference numeral 90. The boiler 90 is substantially identical to the boiler 1, and similar components are identified by the same reference numerals. The main difference between the boiler 90 and the boiler 1 is that it is of a slightly different shape, being of shorter length and wider diameter. While particular sizes, and dimensions of the boiler and its components have been described, the boiler and components could be of other suitable sizes for delivering other quantities of heat. It is envisaged in certain cases that a plug with other shapes and configurations of inner water chambers could be provided, indeed, in certain cases, it is envisaged that the plug may comprise an outer shell only closed at its downstream end by an end cap and the outer shell in combination with the end cap would form an inner water chamber. In which case, it is envisaged that a stand pipe would extend upwardly through the inner water chamber to a position adjacent the dome shaped end wall of the outer shell for drawing water from the inner water chamber adjacent the combustion chamber for delivery into the intermediate water inlet to the outer water chamber. Needless to say, other suitable arrangements of outer water chamber may be provided. It will also be appreciated that it is not essential that the outer water chamber should extend completely around the first heat exchange wall and it will also be appreciated that it is not essential that the outer water chamber should extend completely around the plug. Further, it is not essential that the inner and outer water chambers should extend over the entire axial length of the plug and first heat exchange wall , respectively. Other suitable arrangements of heat exchange fins may be provided and the bands forming the fins may be of other construction and shape besides being of square waveform shape. Indeed, in certain cases the heat exchange fins may be omitted.

Claims: CLAIMS 1. A heat exchanger (5) for a boiler (1), the heat exchanger (5) having an upstream end (3) and a downstream end (4) and comprising a first heat exchange wall (6) extending around and defining an elongated main chamber (7) having an upstream end and an axially spaced apart downstream end corresponding to the upstream and downstream ends (3,4), respectively, of the heat exchanger (5), a hollow elongated plug (17) located in a downstream portion (21) of the main chamber (7) and extending in a generally axial direction therein, the plug (17) comprising a second heat exchange wall (19) forming with the first heat exchange wall (6) a combustion chamber (22) towards the upstream end (3) of the main chamber (7) and an annular flue gas passageway (23) in the downstream portion (21) for accommodating flue gases from the combustion chamber (22) towards the downstream end (4) of the main chamber (7) to a flue gas outlet (31), a mounting means (20) for mounting a burner (76) in the combustion chamber (22), an inner water chamber (36), one wall (18) of which is formed by at least a portion of the second heat exchange wall (19) for transferring heat to water in the inner water chamber (36), an outer water chamber (10), one wall (6) of which is formed by at least a portion of the first heat exchange wall (6) for transferring heat to water in the outer water chamber (10), a return water inlet (42) for delivering water to the heat exchanger (5) to be heated, the return water inlet (42) being provided to the inner water chamber (36) adjacent the downstream end (4) of the heat exchanger (5), a flow water outlet (16) for delivering heated water from the heat exchanger (5), the flow water outlet (16) being provided from the outer water chamber (10) towards the upstream end (3) of the heat exchanger (5), an intermediate water outlet (43) from the inner water chamber (36), an intermediate water inlet (45) to the outer water chamber (10), and a connecting means (46) for connecting the intermediate water outlet (43) to the intermediate water inlet (45) for interconnecting the inner and outer water chambers (36,10) characterised in that the intermediate water outlet (43) from the inner water chamber (36) is located near the combustion chamber (22), and the intermediate water inlet (45) to the outer water chamber (10) is located towards the downstream end (4) of the heat exchanger (5), and the connecting means (46) connects the intermediate water outlet (43) to the intermediate water inlet (45) so that water flows through the inner and outer water chambers (36,10) in series from the return water inlet (42) to the flow water outlet (16). 2. A heat exchanger as claimed in Claim 1 characterised in that the outer water chamber (10) extends completely around the first heat exchange wall (6) . 3. A heat exchanger as claimed in Claim 1 or 2 characterised in that the outer water chamber 10 extends substantially the length of the first heat exchange wall (6) from the upstream end (3) to the downstream end (4) thereof. 4. A heat exchanger as claimed in any preceding claim characterised in that a main outer side wall (9) extends around and is spaced apart from the first heat exchange wall (6) to form with the first heat exchange wall (6) the outer water chamber (10). 5. A heat exchanger as claimed in Claim 4 characterised in that the width of the outer water chamber (10) between the first heat exchange wall (6) and the main outer side wall (9) does not exceed 20 mm. 6. A heat exchanger as claimed in Claim 5 characterised in that the width of the outer water chamber (10) between the first heat exchange wall (6) and the main outer side wall (9) does not exceed 10 mm. 7. A heat exchanger as claimed in Claim 6 characterised in that the width of the outer water chamber (10) between the first heat exchange wall (6) and the main outer side wall (9) does not exceed 5 mm. 8. A heat exchanger as claimed in any of Claims 4 to 7 characterised in that the flow water outlet (16) extends from the main outer side wall (9). 9. A heat exchanger as claimed in any of Claims 4 to 8 characterised in that the intermediate water inlet (45) is provided in the main outer side wall (9). 10. A heat exchanger as claimed in any preceding claim characterised in that the intermediate water inlet (45) to the outer water chamber (10) is located at a position which corresponds to a position in the flue gas passageway (23) at which the temperature of the flue gases is not less than the temperature of the water entering the intermediate water inlet (45) for preventing reverse heat transfer from the water to the flue gases. 11. A heat exchanger as claimed in Claim 10 characterised in that the intermediate water inlet (45) is located at a position intermediate the combustion chamber (22) and the downstream end of the heat exchanger (5) . 12. A heat exchanger as claimed in Claim 10 or 11 characterised in that the intermediate water inlet (45) to the outer water chamber (10) is located nearer to the downstream end (4) of the heat exchanger (5) than to the combustion chamber (22). 13. A heat exchanger as claimed in any preceding claim characterised in that the inner water chamber (36) extends substantially over the entire second heat exchange wall (19). 14. A heat exchanger as claimed in any preceding claim characterised in that the plug (17) comprises an outer shell (18) which forms the second heat exchange wall (19), and an inner shell (35) mounted within the outer shell (18) and spaced apart therefrom and forming with the outer shell (18) the inner water chamber (36) . 15. A heat exchanger as claimed in Claim 14 characterised in that the width of the inner water chamber (36) between the inner and outer shells (36,18) does not exceed 20 mm. 16. A heat exchanger as claimed in Claim 15 characterised in that the width of the inner water chamber (36) between the inner and outer shells (36,18) does not exceed 10 mm. 17. A heat exchanger as claimed in Claim 16 characterised in that the width of the inner water chamber (36) between the inner and outer shells (36,18) does not exceed 5 mm. 18. A heat exchanger as claimed in any of Claims 14 to 17 characterised in that the outer shell (18) of the plug (17) comprises an elongated outer side wall (37) extending around the plug (17) and forming with the first heat exchange wall (6) the flue gas passageway (23), the outer side wall (37) forming an elongated inner cavity (34), and an outer end wall (38) at the upstream end of the outer side wall (37) closing the inner cavity (34) at the upstream end, the outer end wall (38) forming with the first heat exchange wall (6) the combustion chamber (22). 19. A heat exchanger as claimed in Claim 18 characterised in that the inner shell (35) comprises an inner side wall (39) extending around and within the inner cavity (34), and being co¬ axial with and spaced apart from the outer side wall (37), and an inner end wall (40) at the upstream end of the inner side wall (39) which is spaced apart from the outer end wall (38), the inner side wall (39) and inner end wall (40) defining with the outer side wall (37) and outer end wall (38) the inner water chamber (36) . 20. A heat exchanger as claimed in Claim 19 characterised in that the inner and outer end walls (40,38) of the plug (17) are of dome shape, and the intermediate water outlet (43) is located adjacent the centre of the dome. 21. A heat exchanger as claimed in any of Claims 14 to 20 characterised in that the intermediate water outlet (43) is provided from the inner shell (35) of the plug (17). 22. A heat exchanger as claimed in any of Claims 14 to 21 characterised in that the return water inlet (42) is provided in the outer shell (18) of the plug (17). 23. A heat exchanger as claimed in any preceding claim characterised in that a plurality of heat exchange fins (61,53) extend from at least a portion of at least one of the first and second heat exchange walls (6,19) into the flue gas passageway (23) for transferring heat from the flue gases in the flue gas passageway (23) to the at least one of the outer and inner water chambers (10,36) . 24. A heat exchanger as claimed in Claim 23 characterised in that the heat exchange fins (61,53) extend longitudinally in a generally upstream/downstream direction, and are spaced apart circumferentially around the at least one of the first and second heat exchange walls (6,19). 25. A heat exchanger as claimed in Claim 23 or 24 characterised in that the heat exchange fins (61,53) are provided on the first and second heat exchange walls (6,19). 26. A heat exchanger as claimed in any of Claims 23 to 25 characterised in that the heat exchange fins (61,53) extend substantially radially from the corresponding heat exchange walls (6,19). 27. A heat exchanger as claimed in any of Claims 23 to 26 characterised in that the heat exchange fins (61,53) extend from the first and second heat exchange walls (6,19) and terminate in respective distal edges (66,48) at positions intermediate the first and second heat exchange walls (6,19). 28. A heat exchanger as claimed in Claim 27 characterised in that a clearance is provided between the distal edges (66,48) of the respective heat exchange fins (61,53) extending from the opposite heat exchange walls (6,19) to facilitate removal of the plug (17) from the main chamber (7). 29. A heat exchanger as claimed in any of Claims 23 to 28 characterised in that the heat exchange fins (61,53) are arranged so that the sum of the heat exchange surface areas (69,47) of the heat exchange fins (61,53) per unit area of the corresponding first and second heat exchange walls (6,19) increases from the upstream end of the flue gas passageway (23) to the downstream end (4) thereof. 30. A heat exchanger as claimed in Claim 29 characterised in that the sum of the heat exchange surface areas (69,47) of the heat exchange fins (61,53) per unit area of the corresponding first and second heat exchange walls (6,19) increases from the upstream end of the flue gas passageway (23) to the downstream end thereof in a way that the quantity of heat being transferred to the respective first and second heat exchange walls (6,19) is substantially constant along the length of the respective first and second heat exchange walls (6,19) from the upstream end thereof to the downstream end thereof. 31. A heat exchanger as claimed in Claim 29 or 30 characterised in that the number of the heat exchange fins (61,53) per unit area of the corresponding first and second heat exchange walls (6,19) increases from the upstream end of the flue gas passageway (23) to the downstream end thereof. 32. A heat exchanger as claimed in Claim 31 characterised in that the number of heat exchange fins (61,53) per unit area of the corresponding first and second heat exchange walls (6,19) increases at intervals in a downstream direction along the flue gas passageway. 33. A heat exchanger as claimed in any of Claims 23 to 32 characterised in that the heat exchange fins (53,61) are arranged in sets (52,60), each set (52,60) comprising a plurality of heat exchange fins (53,61) spaced apart circumferentially around the flue gas passageway (23), the sets (52,60) of heat exchange fins (53,61) being arranged end to end in a generally upstream/downstream direction. 34. A heat exchanger as claimed in Claim 33 characterised in that the heat exchange fins (53,61) of at least some of the sets (52,60) of heat exchange fins (53,61) are staggered relative to the heat exchange fins (53,61) of an adjacent set (52,60) of heat exchange fins (53,61) to induce turbulence in the flue gases passing through the flue gas passageway (23). 35. A heat exchanger as claimed in Claim 33 or 34 characterised in that each set (52,60) of heat exchange fins (53,61) is formed by an elongated endless band (54,63) of heat conductive material, each band (54,63) being formed into a waveform, portions (55) of the waveform joining opposite peaks (59) of the waveform forming the heat exchange fins (53,61). 36. A heat exchanger as claimed in Claim 35 characterised in that each band (54,63) is of square wave formation, the heat exchange fins (53,61) being formed by radially extending portions (55) of the square wave. 37. A heat exchanger as claimed in Claim 35 or 36 characterised in that the bands (63,54) are secured to the respective heat exchange surfaces (6,19) by the peaks (56,59) joining roots of the waveform. 38. A heat exchanger as claimed in any preceding claim characterised in that the first heat exchange wall (6) and the plug (17) are of cylindrical construction. 39. A heat exchanger as claimed in any preceding claim characterised in that the plug (17) is located co-axially within the first heat exchange wall (6). 40. A boiler (1) comprising the heat exchanger (5) according to any preceding claim. 41. A boiler (1) as claimed in Claim 40 characterised in that the boiler (1) is a condensing boiler. 42. A boiler (1) as claimed in Claim 40 or 41 characterised in that the boiler is gas fired. 43. A boiler (1) as claimed in any of Claims 40 to 42 characterised in that the boiler is suitable for mounting with the upstream end (3) of the heat exchanger towards the top, and the downstream end (4) of the heat exchanger towards the bottom. 44. A boiler (1) as claimed in any of Claims 40 to 43 characterised in that the burner (76) is mounted in the combustion chamber adjacent the upstream end thereof.

Inventors: AHERN JOHN MICHAEL, CONNEELY THOMAS, COSTELLOE CIARAN JOHN, MONAGHAN PAUL, VAUGHAN MICHAEL, WOOD JOHN, AHERN, JOHN, MICHAEL, CONNEELY, THOMAS, COSTELLOE, CIARAN, JOHN, MONAGHAN, PAUL, VAUGHAN, MICHAEL, WOOD, JOHN
Applicants: AHERN JOHN MICHAEL, CONNEELY THOMAS, COSTELLOE CIARAN JOHN, MONAGHAN PAUL, RUSTSUN LIMITED, VAUGHAN MICHAEL, WOOD JOHN, AHERN, JOHN, MICHAEL, CONNEELY, THOMAS, COSTELLOE, CIARAN, JOHN, MONAGHAN, PAUL, VAUGHAN, MICHAEL, WOOD, JOHN
Application year: 1994
Main classification: F24H1
IPC codes: F24H1, F24H9